Electrostatic discharge protection structures with reduced latch-up risks

ABSTRACT

The present invention provides an ESD protection circuitry in a semiconductor integrated circuit (IC) having protected circuitry to prevent false triggering of the ESD clamp. The circuitry includes an SCR as an ESD clamp having an anode adapted for coupling to a first voltage source, and a cathode adapted for coupling to a second voltage source. The circuitry also includes at least one noise current buffer (NCB) coupled between at least one of a first trigger tap of the SCR and the first voltage source such that the first trigger tap of the SCR is coupled to a power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/808,041 filed on May 23, 2006, contents of which are incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to the field of electrostatic discharge(ESD) protection circuitry and, more specifically, improvements inpreventing false triggering of silicon controlled rectifier (SCR)circuits in the protection circuitry of the integrated circuit (IC).

BACKGROUND OF THE INVENTION

In the prior art, in order to prevent false triggering of the ESD clamp,the noise is modeled by voltage overshoots. For voltage overshoots, aNoise Voltage Buffer (NVB) is used as can be seen in FIG. 1. FIG. 1shows a schematic block diagram of the prior art representing an ESDprotection circuit 100 of an integrated circuit (IC). The ESD protectioncircuit 100 includes an ESD clamp, a SCR 102 coupled to a Node1 104which represents a pad of the IC. The IC pad of Node1 104 may be aninput pad, an output pad, or a supply pad. Node2 106 may be a ground oralso an input/output pad. The triggering tap, G2 of the SCR 102 iscoupled to Node3 108 which may be an input pad, an output pad or asupply pad. In this example of FIG. 1, Node1 104 represents an inputpad, Node2 represents ground and Node3 108 represents a power supply. ANoise Voltage Buffer (NVB) 110 is coupled between the triggering tap G2of the SCR 102 and Node3 108. A shunt resistor R1 112 is optionallycoupled between the SCR 102 and the Node2 106.

During normal operation, Node3 108 is powered, however at Node1 104 thevoltage is lower than the power supply, i.e. not enough voltage toconduct current between the anode and the grounded cathode. Thus, theSCR 102 is turned off. In order for SCR 102 to turn on there must be atleast 0.7 volts between Node1 and G2 of the SCR 102. Because the inputvoltage, i.e. at Node1 104 is below the power supply, i.e. Node3 108,thus SCR 102 cannot trigger during normal operation. During ESD, thepower supply at Node3 108 is essentially at 0 volts, however, thevoltage at Node1 104 is high, i.e. at least 0.7 volts or higher, thenthere will be voltage over G2 anode junction, causing the SCR 102 toturn on or trigger. The ESD current will run through the SCR 102 fromNode1 104 to Node2 106.

In a case scenario the input voltage at Node1 104 may become higher thanthe power supply, Node3 108 during normal operation. For example voltageat Node3 108 is 1.8 volts and voltage at Node1 102 is 2 volts or higherwhich can trigger the SCR 102 to turn on during normal operation.Normally the voltage at the input or output node 104 is limited belowthe power supply, but voltage overshoot (noise, spikes) can introducethese overvoltages. Thus, in this situation, SCR 102 is triggered notdue to the ESD, but due to high voltage at the input pad, Node1 104.This is the false triggering of the SCR 102 which is not a desiredapplication during normal operation. Thus, the NVB 110 will lower thevoltage occurring between the anode and G2 by dividing the voltage inseries. So, for example, the 0.7 volts at Node1, will be divided into0.3 volts over the G2 anode junction and 0.3 volts over the NVB 110,thus limiting the voltage over the G2 anode junction. Therefore, NVB 110prevents G2 from the triggering of the SCR 102 during normal operation,thus preventing that the voltage overshoot noise will trigger the SCR.

Although attempts have been made in the past to reduce the falsetriggering of the SCR by different circuit techniques, there still exista danger for unwanted triggering of the device during normal supply linepowered operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an illustration of a block diagram of a prior artimplementation of an ESD protection circuit.

FIG. 2 depicts an illustration of a block diagram of an ESD protectioncircuit having a Noise Current Buffer in accordance with one embodimentof the present invention.

FIG. 3 depicts an illustration of a block diagram of the ESD protectioncircuit of FIG. 2 with a combination of a Noise Current Buffer and aNoise Voltage buffer in accordance with another embodiment of thepresent invention.

FIG. 4A depicts an illustration of a block diagram of FIG. 2 with acombination of Noise Current Buffer and Leakage Buffer in accordancewith even further embodiment of the present invention.

FIG. 4B depicts an illustration of a block diagram of FIG. 2 with acombination of Noise Current Buffer and Leakage Buffer in accordancewith an alternate embodiment of the present invention.

FIG. 4C depicts an illustration of an exemplary implementation of theNoise current Buffer and Leakage Buffer of FIG. 4A in accordance with apreferred embodiment of the present invention.

FIG. 4D depicts an illustration of an another exemplary implementationof the Noise current Buffer and Leakage Buffer of FIG. 4A in accordancewith a preferred embodiment of the present invention.

FIG. 5A depicts an illustration of a block diagram of FIG. 2 with acombination of Noise Current Buffer, and Noise Current Margin Increaserin accordance with an alternate embodiment of the present invention.

FIG. 5B depicts an illustration of an exemplary implementation of theNoise Current Buffer, and Noise Current Margin Increaser of FIG. 5A andthe Leakage Buffer in accordance with a preferred embodiment of thepresent invention.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided anelectrostatic discharge (ESD) protection circuit in a semiconductorintegrated circuit (IC) having protected circuitry. The ESD protectioncircuit comprise a first voltage source of a protected circuit node ofthe IC, a silicon control rectifier (SCR) having an anode adapted forcoupling to the first voltage source, and a cathode adapted for couplingto a second voltage source. The circuit further comprises at least onenoise current buffer (NCB) coupled between at least one of a firsttrigger tap of the SCR and the first voltage source, wherein the atleast one of the first trigger tap of the SCR is coupled to a thirdvoltage source.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is modeled to prevent that the SCR will triggerdue to uncontrolled current in the input of the ESD clamp during normaloperation. Excessive current can cause latch-up if it is injected in theESD clamp. When enough current flows into the anode-G2 junction of theSCR 102, the SCR 102 will trigger or turn on which is not ideal for thenormal operation. This problem of false triggering of the SCR 102 duringnormal operation is solved by adding a Noise Current Buffer (NCB) 202 tothe ESD protection circuit 200 as shown in FIG. 2. Note that the NCB 202is placed in parallel with the trigger node of the ESD protectionbetween the Anode-G2 junction of the SCR 102. So, the current is nowdivided into two parallel paths.

During normal operation, Node 3 108 is powered and the voltage at Node1104 is between ground and the power supply. Injected current at Node1104 will now flow mainly through the Noise current buffer (NCB) 202 andonly a very small part will flow through the Anode G2 junction. Thispart is too small to trigger the SCR 102. The main part of the currentcan now flow towards Node3 108.

During ESD, which itself is a current injector, the current will flowthrough both the anode-G2 junction and the NCB 202, thus requiring morecurrent to turn on the SCR 102 (since only the current flowing inanode-G2 junction will turn on the SCR 102). The Anode G2 junction ofthe SCR 102 can be designed to handle a certain amount/level of currentduring the triggering. Once the level of current is high enough, the SCR102 will turn on. During normal operation, the current value rangesbetween 0 mA and 100-200 mA. However, during ESD, the current value canbe high, ranging in 0 A and 2-3 A. Thus, a certain level of threshold incurrent value is provided for NCB 202. The threshold may rangepreferably from 0.1 A to 0.3 A. Below this current value the SCR 102will not trigger, but if the injected current becomes higher than thisvalue the SCR can trigger.

FIG. 3 depicts an illustration of a block diagram of the ESD protectioncircuit 200 of FIG. 2 with a Noise Voltage Buffer (NVB) 110 inaccordance with another embodiment of the present invention. Theadvantage of using both techniques is that the ESD clamp can be madesafely for voltage overshoots/noise and also for currentovershoots/noise during normal operation. The operation of this circuitis the same as discussed above with respect to FIGS. 1 and 2

Since the NCB 202 might introduce high leakage current (undesirableleakage) between Node3 108 and Node1 104, a Leakage Buffer (LB) 204 maypreferably be added to the ESD protection circuit of FIG. 2. The leakagebuffer (LB) 204 will prevent the current flow between Node1 104 andNode3 108 during normal operation. The need of the LB 204 depends on theimplementation of the noise current buffer, NCB 202, i.e. it depends ifthe NCB 202 is in conduction during normal operation. The LB 204 can beplaced either in series with the ESD trigger node G2 as shown in oneembodiment in FIG. 4A, or in parallel with the ESD trigger node G2 asshown in another embodiment in FIG. 4B.

The trigger voltage in FIG. 4A preferably comprise of 0.7 volts ofG2-anode junction, plus the voltage over the LB 204 to conduct current.(The advantage of placing the leakage buffer (LB) 204 in parallel inFIG. 4B is that the trigger voltage during ESD does not include thevoltage over the LB 204. This will give a clamp with a lower triggervoltage and so a better clamping device to protect Node1 104. Thus,trigger voltage in FIG. 4B is simply the 0.7 volts of G2-anode junction,since the LB 204 is not in series with the trigger node, G2, so thetrigger voltage is much lower. Although, not shown, a NVB 110 can beconnected between the trigger node G2 and the Node3 108 of the ESDprotection circuit of FIG. 4A and FIG. 4B if extra protection is neededfor voltage overshoots/noise.

Referring to FIG. 4C, there is show an exemplary implementation of theNoise Current Buffer (NCB) 202 and the Leakage Buffer (LB) 204 of FIG.4A in accordance with a preferred embodiment of the present invention.In this embodiment, the NCB 202 consists of a resistor 402 and the LB204 consists of a diode 404. Note that the Noise immunity is now˜0.7V/R_(NCB). This value is the minimum current needed for triggeringthe SCR 102. If the maximum current is injected during normal operationif for example 100 mA, the needed resistance value for the resistor 402can be calculated with the formula. This resistance value provides thecertain level of threshold in current value for the NCB 202. Also, thediode 404 (acting as a leakage buffer 204) will prevent the current flow(i.e. the leakage current) between Node1 104 and Node3 108 when theresistor 402 is in conduction during normal operation

Furthermore, FIG. 4D depicts an illustration of an another exemplaryimplementation of the Noise current Buffer (NCB) 202 and Leakage Buffer(LB) 204 of FIG. 4A in accordance with a preferred embodiment of thepresent invention. In this embodiment, the LB 204 consists of the diode404 similar to FIG. 4C, however, the NCB 202 consists of an activeelement, NMOS 406. As shown in FIG. 4C, the source of the NMOS 406 isconnected to the anode of the SCR 102 and the gate of the NMOS 406 isconnected to its drain and the drain is further connected to G2-anodejunction. Since the voltage at Node3 108 is smaller than the voltage atNode1 104 during ESD, the gate voltage of the NMOS is low, thus the NMOS406 is turned off. When the NMOS 406 is turned off, it is highlyresistive, i.e. a large amount of current will flow through the NMOS 406which further flows into the anode-G2 junction, thus making it quiteeasy to turn on the SCR 102.

During normal operation, the voltage at Node3 108 is higher then thevoltage at Node1 104, thus the sate voltage of NMOS 406 is high, whichturns on the NMOS 406. When NMOS 406 is turned on, it is low ohmic, i.e.small amount of current will flow through it and into the anode-G2junction. This low amount of current is not enough to trigger on the SCR102, thus preventing the SCR 102 to turn on during normal operation.During normal operation the NMOS 406 is turned on, thus current willflow from Node3 10S to Node1 104. This leakage is undesirable, thus a LB204 is needed to block this leakage. Which is in this case is a diode404 During normal operation the diode is in reverse thus blocking thecurrent flow from Node3 108 to Node1 104.

Although in the above examples of FIG. 4C and FIG. 4D, the NCB 202 isillustrated as a resistor and NMOS respectively, and LB 204 is shown asdiodes, it is important to note, that in general they can consist of anyactive elements such as NMOS, PMOS, bipolar transistor, diode, orpassive elements such as resistor, metal, inductor, capacitor. Thus, thescope of the invention is not limited to the use of a specific elementas NCB and LB.

Since the ESD clamp is an SCR 102 in the embodiments as shown above, theNCB 202 must work below 0.7V (25° C.) to avoid triggering of the clamp.However, in order for the SCR 102 to trigger, it requires at least 0.7Vand higher. Thus, another element such as Noise Current Margin Increaser(NCMI) 502 is added to the ESD protection circuit as shown in FIG. 5A ifthe NCB 202 works during normal operation above 0.7V. The Noise CurrentMargin Increaser (NCMI) 502 is added to increase the voltage designspace of the NCB 202. The NCMI 502 is placed in parallel with the NCB202. If the NCB 202 is a resistor, for example, a large resistor, thetrigger voltage can be increased by the NCMI 502. The voltage over theNCB 202 would be 0.7 volts plus the voltage over the NCMI 502. Thus,more current injected voltage is now required to trigger the SCR 102.This way by adding NCMI 502, the NCB 202 for example as a resistor canpreferably have a higher value and the latch-up immunity will be thesame (same current as in previous cases). With the same current injectedthe voltage over NCB 202 will be higher, but the clamp, SCR 102 will nottrigger because the extra voltage will be over the NCMI 502 and not overthe Anode-G2 junction. Although, not shown in FIG. 5A, the LB 204 may bepreferably be placed either in parallel or in series with the NCMI 502.Similarly, the NVB 110 may desirably be placed in series with the NCMI502.

Referring to FIG. 5B, there is illustrated an exemplary implementationof the Noise Current Buffer, and Noise Current Margin Increaser of FIG.5A with the addition of the LB 204 in accordance with a preferredembodiment of the present invention. In this embodiment, the NCB 202consists of a resistor 402, the LB 204 consists of a diode 404 and theNCMI 502 also consists of a diode 504. Note that the noise immunity(maximum current) is now ˜1.4V/R_(NCB). This value is doubled comparedto the build in voltage (0.7V) of the diode 404 in the circuit of FIG.4C without the NCMI 502. This is especially useful for high temperatureapplications, since the diode built-in voltage drops for highertemperatures (˜0.4V for 100° C.), and therefore the voltage design spacedrops.

Although in the given example of FIG. 5B, the NCMI 502 and LB 204 areshown as diodes, it is important to note, that in general they canconsist of any active elements such as NMOS, PMOS, bipolar transistor,diode, or passive element such as resistor, metal, inductor, capacitor.Thus, the scope of the invention is not limited to the use of a specificelement as NCMI and LB.

Although various embodiments that incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings without departing from the spirit andthe scope of the invention.

1. An electrostatic discharge (ESD) protection circuit in a semiconductor integrated circuit (IC) having protected circuitry, the ESD protection circuit comprising: a first voltage source of a protected circuit node of the IC; a silicon control rectifier (SCR) having an anode adapted for coupling to the first voltage source, and a cathode adapted for coupling to a second voltage source; and at least one noise current buffer (NCB) coupled between at least one of a first trigger tap of the SCR and the first voltage source, said at least one of the first trigger tap of the SCR is coupled to a third voltage source.
 2. The ESD protection circuit of claim 1 wherein said NCB functions to prevent triggering of the SCR during normal operation.
 3. The ESD protection circuit of claim 1 wherein said NCB is a resistor.
 4. The ESD protection circuit of claim 1 wherein said NCB is a MOS transistor.
 5. The ESD protection circuit of claim 1 wherein said third voltage source is a power supply.
 6. The ESD protection circuit of claim 1 wherein said second voltage source is a ground.
 7. The ESD protection circuit of claim 1 further comprising a leakage buffer (LB) coupled between the NCB and the third voltage source.
 8. The ESD protection circuit of claim 7 wherein said LB functions to prevent the current flow between the first voltage source and the third voltage source during normal operation.
 9. The ESD protection circuit of claim 7 wherein said LB is coupled in series between the at least one of the first trigger tap of the SCR and the third voltage source.
 10. The ESD protection circuit of claim 7 wherein said LB is coupled between NCB and first trigger tap of the SCR in parallel with said at least one of the first trigger tap of the SCR and the first voltage source.
 11. The ESD protection circuit of claim 7 wherein said LB is a diode.
 12. The ESD protection circuit of claim 11 wherein said diode is coupled in reverse from third voltage source and the NCB.
 13. The ESD protection circuit of claim 1 further comprising a noise current margin increaser (NCMI) coupled in series with said at least one of the first trigger tap of the SCR and the third voltage source.
 14. The ESD protection circuit of claim 13 wherein said NCMI functions to increase the voltage design space of the NCB.
 15. The ESD protection circuit of claim 13 wherein said NCB is coupled in parallel with the NCMI.
 16. The ESD protection circuit of claim 13 wherein said NCMI is a diode.
 17. The ESD protection circuit of claim 1 further comprising a noise voltage buffer (NVB) coupled between said at least one of the first trigger tap of the SCR and the third voltage source. 